Heart Rate Decrease During exercise

I've been training at a competitive level for some time- Recently during first 30 minutes (total 90 minute 10 mile 6 days per week) of workout my heart rate will hit 160 bpm- which I can tolerate normally.  (BTW: 37 Yo healthy male- no history)...
Taking in plenty of fluid, typically a mixture of Gatorade and water.  At about 25 minutes felt discomfort in stomach, ignored, but checked pulse.  HR decreased to 90 bpm- should have been 160...This continued for another 3-4 minutes eventually with enough discomfort that I slowed the pace down, and everything seemed to go back to normal...
I typically wear sweatpants, and hooded sweatshirt to produce an increase in the heat and sweating threshold.  Sweating more than adequately!
Was this an electrolyte problem or ???